As is well known, vacuum cleaners use an electric motor driven fan unit to generate suction which draws dirt-laden air through a dirty air inlet into the body of the vacuum cleaner. The dirty air passes through a separating apparatus, either in the form of a bag or a cyclonic separation system, in order to separate the dirt particles from the air flow.
Whether the separation apparatus involves a bagged or a ‘bagless’ system, the vacuum cleaner must be emptied when it is full. Continued operation of the vacuum cleaner when it is in a full state may impair its performance or may cause blockages in other parts of the vacuum cleaner so prompt attention is desirable. In some machines it is left up to the user to determine whether the bag or bin is full, although some machines incorporate a suitable mechanism to indicate to a user when the vacuum cleaner needs to be emptied. A similar issue can arise with filters which become blocked. Over-loaded filters present an undesirable resistance to airflow and a resultant loss of airflow power. Therefore it is desirable to identify a fault condition with a filter promptly in order to encourage filter maintenance in a timely manner.
In a known system, documented in WO01/28401, an indication that the vacuum cleaner is blocked is determined on the basis of monitoring the speed of the fan motor. Here, a control unit monitors a speed or power draw parameter of the fan motor and triggers a warning indicator when the parameter exceeds a preset value for a predetermined period of time. In addition to triggering a warning indicator, the suction fan may also be disabled as a further response to the overspeed condition. The benefit of such a system is that it is capable of determining if the vacuum cleaner is blocked merely by monitoring the speed of the suction fan motor with existing hardware. It does not therefore require additional electronic or mechanical means of monitoring the blockage condition of the machine so it is a cost effective system. However, in practice such a diagnostic method lacks robustness across a range of operating conditions of the vacuum cleaner so its usefulness is limited.
A more practical system for monitoring a fault condition of a vacuum cleaner, and air moving appliances more generally, is required.